Electromagnetic transducer



lunes, 1942. WESTEERS 42,286,123

ELECTROMAGNETIC TRANSDUCER Filed May 29, 19156V K Arrow/Vey Patented June 9, 1942 i- William F. Steers, Audubon, N. J., lassignor tov Radio Corporation of America, a corporation of Delaware Application May 29, 1936, Serial No. 82,579

3 Claims. My invention relates to improvements injelectro-magnetic transducers, and more specifically to electro-dynamic loud speakers, as of the moving coil or induction type, having a magnetic eld structure energized by iiuctuating orl pulsating direct current.

In electro-dynamic loud speakers, in general` use, audio'frequen'cysignal currents, as from the output of a radio receiving circuit, are translated into sound waves bythe magnetic interaction between a moving or voice coil carrying said currents, attached to a sound producing diaphragin, and an intense magnetic eld. Con-i siderable diiulty has been encountered from the introduction of disturbing fluctuations, or

hum, into the moving coil and sound output. This has been caused by the use of fluctuating andarranged in an inverse sense with respect to the source of h um disturbance with the purpose of obtaining a 180 degree phase differ--l ence, or exact phase opposition, for proper neutralization. The difficulty has been that the desired phase opposition vhas not resulted, more particularly in the case of higher frequencies, such as the higher harmonics of sixty cycle power supply hum. For additional reasons, the effectiveness. of the neutralizing coil has fallen off at the higher frequencies; in fact theneutralizing coil has been found to actually introduce hum disturbances, in the high frequency current for` energizing the eld coil, as when uni-directional current has been derived, as by means of a thermionic rectifler, from alternating current powerV supply mains. In cases .where the eld has been energized from direct current mains, wherein alternating current in the windings ofa generator at the power house is rectied mechanically by a commutator, the resulting commutator ripples or pulsations have also caused problems of a somewhat different, although none the less serious, nature.

Several Well known devices havebeen used in an attempt to eliminate the disturbances, but with incomplete success. One `arrangement heretofore employed consistedyin a massive damping or shading ring of low resistance nonmagnetic material,I such as copper, disposed about the center pole o f an. electro-dynamic speaker, betweenthe field coil and the voice coll. Space limitations and cost of material have, however, in practice dictated a ring of moderatedimensions for a given speaker design, but'I have found that such a ring-is in-l sufficient to effect the required reduction oiuctuation disturbances in the lowerrr Yaudio range. VV'l'he shading ring,'I have found, is quite effective in the reductionof the higher frequency eiects from commutator ripple in the neld magnet, a serious disturbance being of the end of the range, that would not be there in the absence of the coil, as will be seen later.

It is,` accordingly, an object of my invention to provide an effective arrangement for eliminating the disturbing effects of a iiuctuating magnetic field in an electro-magnetic transducer, par- 4 ticularly an electro-dynamic speaker having a field Winding energizedby unidirectional current with a fluctuating component, in other words, an

incompletely filtered current.A

It is a further object of myinventin to provide an electro-dynamic speaker, having a field coil, with a new combination of elements making the speaker output `substantially free from objectionable power hum disturbances, regardlessofv whether the field is energized from A. C. mains through a rectiiier, or from D. C. mains, as inthe case of a'radio receiver known asv the universal type, that is one adapted to be plugged at will into either A. C. or D. C. mains.

Inaccordance with my invention I have found that the combination of different methods and means of hum suppression, having complementary characteristics over the audio frequency range in anelectrov-dynamic speaker, gives new andunexpeted results in the way of eliminationof fluctuating disturbancesbf Kall'frequen- My invention will be readily understood from the following description in connection with order of 1,000 cycles, as well as the effects -from the higher harmonics of ordinary rectified a1- ternating current from house mains.

. Another-'arrangement in quite general use has consisted in aneutralizing or hum-buckingcoil connected in circuit with the voice coil and disposed around the center pole, between the voice coil and .the ield coil.' The bucking coil has i been connected in series with the voice coil the figures of the accompanying drawing in which like reference charactersrepresent like elements in the several figures.

Figure 1 is a side elevational view, partly in section, of an electro-dynamic liudspeaker including lfeatures of my "invention, ..Fig. 2 is a schematic electrical diagram illus trating vthe manner of connection ofthe electrical elements, in the speaker shown in Fig. l, Figs. 3 and 4'show characteristics curves representing an'operating condition of an electrotuations or unfiltered pulsations.

dynamic loudspeaker, both with and without the features of my invention, and

Figs. and 6 are fragmentary side elevational views, partly in section, of modifications embodying my invention.

Referring to Fig.'1, the electro-dynamic loudspeaker comprises a magnetic field structure -formed by a U-shape yoke I of magnetically permeable material, such as iron or mild steel, an inner pole formed of similar material and including a core base 3 and a separate pole piece or core end 5, also of similar material, the core 3 and end 5 being fastened in place on the yoke I as by means of a bolt 'l that extendsy axially through a bore in the core 3 and end 5.

. The parts areclamped in position by nut 8.

An annular pole piece or yoke plate 9 is secured to the ends of yoke I by suitable means such as resistance welding, shown at II. The plate is provided with a circular opening forms with the inner pole piece 5 a magnetic air gap in which a voice coil I5 and form I2 secured to the small end of a frusto-conical diaphragm I6 are adapted to vibrate in an axial direction. A spacing or centering ring I1, of non-magnetic material, such as brass, is secured to the yoke plate, as by welding or other known method, and serves to accurately space the. pole piece 5 and yoke plate opening I3 to form a uniform magnetic gap. The yoke plate is attached to the yoke by means of the interposition of the small flange I9 of a diaphragm housing or dish-pan adapted to support the base of the diaphragm I6, a portion only of which has been shown.

A field coil 23 is shown around the inner pole for supplying the operating flux for the magnetic circuit of the speaker. This coil consists of many turns of small wire through which ows a unidirectional current derived ordinarily from a rectified alternating eurent, and containing fluc- This is particularly the case when the eld is used as a filter choke in a power supply as disclosed by Rice and Kellogg Patent 1,728,879.

Referring to Fig. 1, a new method of assembling the field magnet is shown. During manufacture the cone housing I8, yoke plate 9, and yoke I -are fastened together by a -single operation resistance welding process: Projections which are embossed into the cone housing between adjacent surfaces of the yoke and yoke' plate, melt and solidify in union -with the tw\ magnet partsas a current and pressure is ap plied between the yoke plate and yoke. The hum neutralizing 2l and 29 are assembled over the outside of the core end centering ring I1; the

` .field coil 23, core 3, and clamping washer 24 are arises where the field coil needs to be replaced.

A centering spider 25 is attached to the small end of the cone and extends to a bracket 26, as shown byk way of example, on the cone housing I9 for centering the voice coil in the air gap.

A neutralizing coil 2l, of about twentyfive turns, is connected in series with the voice coil I5 to reduce resistance in the voice coil circuit.

In the design of a speaker, adjustment of the number of turns in the neutralizing coil is made so that the resultant hum voltage at 120v cycles is a minimum. To take care of variations in production it is desirable to have a satisfactory factory adjustment on each speaker. Accordingly, a preferred method for production is to wind the coil with a number of turns greater than necessary for neutralization at 120 cycles;

a wire of high specific resistance is then connected to the terminals in shuntwith the coil, and its effective length determined by any convenient method to reduce the inductive reactance of the coil to the right amount for neutralization. Referring to Fig. 2, as an example of one way of obtaining the desired adjustment, the ends of the resistance wire 28 may be Soldered to the terminals of the coil, an intermediate portion twisted to reduce the resistance to` the desired value, and the twisted part then soldered to x the resistance. more accurate results in hum reduction than merely adjusting the actual number of turns,

and compensates for the production tolerancesv of the coils and more particularly, the variation of the location of the voice coil in the air. gap among various speakers in production.

According to my invention I have found that a neutralizing coil alone is more effective for the suppression of the effect of the low frequency disturbances on the voice coil, while the shading ring, considered alone, is more effective than the neutralizing coil for the suppression of higher frequency disturbances. I have found that new and highly desirable over-al1 results are obtained. by combining these two means, that have complementary characteristics, so as to effectively reduce both low frequency and high frequency disturbances. Furthermore, I found that an unusual result additionally occurred that would not be expected from the combination of the two means, namely, that the presence of the shading ring improved the action of the neutralizing coil in performing its own function better. These new and unobvious results may be more clearly understood from an insection of the curves shown in Figs 3 and 4.

More specifically in accordance with my invention, a small fiat shading ring 29 of proper dimensions and of low resistance material, such as copper, is positioned between' the field coil of the electro-dynamic speaker and the neutralizing ceil. The .shading ring is of non-magnetic material and, therefore, `does not reduce the steady uni-directional flux lin'es in the air gap in -any way' whatsoever. The leakage ux in the.

.magnet is not increased and the working fiux in the air-gap is not'decreased, as `would happen if the shading ring were made of magnetic material. The-copper ring reduces the coupling between the field coil and the voice coil and neutralizing coil and, furthermore, maintains the coil voltage Ev approximately equal to the neu- This method gives tralizing voltage En as the frequency is varied. Also, the copper ring reduces the phase angle between Ev and En; consequently, the resultant hum voltage En is greatly reduced at 120 cycles; as a .result o f improved neutralization, and is kept constant, or decreases with frequency. this combination, the copper .ring is smaller in volume than is ordinarily the case, being only one-sixteenth inch thick and about two inches in diameter in the present case, with the result that its cost is-low and its effect on the reduction of the impedance of the field coil ,is'negliglbla Referring to Fig. 3 a series of curves'are shown plotted between frequency, in cycles per` second,

and decibels corresponding to hum or fluctuating current disturbances. The horizontal line at zero decibels corresponds to the hum in the speaker voice-coil without any hum reducing means. Curve A shows the hum characteristic using a damping ring alone, it being noted that the reduction in hum increases as the frequency increases. Curve B shows the hum characteristic using a neutralizing coil alone, it being observed that the ihum increases with frequency; it will be seen from curve B that the hum in the higher audio range, where it crosses the zero line, is actually greater than if no neutralizing coil were used. Curve C shows va resultant effect that might be expected from the sum of the curves of my new use of the two hum reducing means. Curve D shows the actual hum characteristic using both the damping ring and the neutralizing coil in accordance` withmy invention, it being observed that the reduction in hum is substantially for low and high and intermediate frequencies throughout a wide range. The difference between curves C and D illustrates the beneficial effect of the shading ring on the neutralizing coil in causing better neutralization,

-per se.

Referring to Fig. 4, a series of curves are plotted between frequency in cycles per second and phase angle in degrees as well as voltage of certain of the elements. Curve F shows how the hum voltage induced in the voice coil varies with frefquency, using the neutralizing coil-only. Curve H shows how the hum voltage induced in the neutralizing coil varies with frequency using the neutralizing coil only. Curve K- represents the resultant hum voltage Eh, using the neutralizing coil only. Curve L represents the hum volt- `tage Eh under the condition where boththev neutralizing coil and the shadingring are used. Curve G shows thevariation in hum voltagejn lthe voice coil when using both the neutralizingl coil and shading ring. Curve J shows the variation of hum voltage in the neutralizing coil, using both theneutralizing coil and the shading ring. Curve M shows the variation in phase angle between the hum voltage Ev in the voice coil and the hum voltage En in the neutralizing coil using `the neutralizing coil o'rlly- Curve N shows the variation of the phase angle between the voltageV Ev in the voice coil and the voltage En in the neutralizing coil under the condition of use of the neutralizing cil in combination with. the shading ring. It will be seen, therefore, that varies widely over thel hum frequency range..

across the air gap containing the voice coil and it thus links both the voice coil and the neutralizing coil; The other part passes from the pole piece 5 to the annular yoke piece 9 through the air space between the two. This second part, the leakage flux, does not link with the voice coil but it does link with the neutralizing coil.w The hum voltage induced in the voice coil is* produced by the rst part of the flux. The counter hum voltage in the neutralizing coil, however, is produced by both the rst and second parts of the fiux. might not be so lpbjectionable if the relative strengths of the two fluxes were constant'and had the same wave form over the hum 4 frequency range, but this is not the case. ThereA is a Wide variation with the frequency of the uctuating field current. Therefore the hum re-v duction by the neutralizing coil, when used alone,

When my copper ring is p laced against the neutralizing coil as shown, however, any leakage flux linking with the neutralizing coil must also link with the ring, Currents thereby induced in the ring act to oppose uctuation of the leakage ux linking with both the ring and coil; in other4 words, the ring shields or shades the coil from leakage fiux fluctuations,

By constructing the shading ring so that it is at fleast ccextensive with the neutralizing coil, is

adjacent thereto and is located between it and the eld coil, as 'shown in Figure l, the disturbing v leakage flux is substantially eliminated and the hum reduction does not materially vary over the frequency range. If the shading ring were of smaller diameter and did not cover the entire side of the neutralizing coil facing the eld coil, some ofthe second part of the ux would cut the neu-l tralizing coil without cutting the shading ring and a disturbing voltage would be induced in the neutralizing coil and the desired balanceover the frequency range would. not be obtained.

Referring to Fig. 5, I have shown a modified form of myinvention wherein, for more complete neutralization, the neutralizing coil 21 is positioned closer tothe air gap. Whereas in Fig.

1 the brass spacing ring is interposed between `the gap and the neutralizing coil for manufacturing reasons In Fig. 5 I haveshown the coil 21"4 between the centering ring I8 and the gap. While a shading ring similar to tha-t in Fig. 1, may be employed below the usual centering ring, for

. simplification of parts I have chosen to so design the combination of the shading ring with the rent maybe considered as comprising two parts. One part of the fux, the useful ux, passes a spacing ring as to function effectively as a shading ring. For this purpose it ismade of low resistance material, with sufficient cross section,

such as phosphor vbronze orcopper.

While I have above illustrated my invention in connection with the moving coil type of speaker wherein the signaling currents are fed directly to the voice coil, I desire it to be understood that the invention may be applied to other types, such as the induction type as shown, for example, in

the patents to Field 540,969, and Arkell 1,743,749,

and Nordenswan, 1,644,789. I have illustrated one form of this type in Fig. 6, wherein the signal currents are supplied to a stationary coil -20 mounted on the field-structure, and transferred by induction to a closed conductor I4 disposed in the airgap. I desire to employ the term armature to broadlyrefer, to the movingelement Il and l 2 connected to the diaphragm, which in this case includes the closed conductor and in the other cases -the ordinary voice coil. In Fig. 6

the stationary coil may be termed the voice coil and the armature conductor may be classed as a being so poled that the voltages induced-,therein by the rst part of said flux are in opposition, and a low resistance metal ring for shielding said neutralizing coil from said otherpart of the ilux of my invention an electro-dynamic moving coil loud speaker, I wish it to be understood thatmy invention may be employed in connection with other electrical-l transducers, such as dynamic microphones, phonograph pickups, etc. As an example of such `a microphone I may refer to Hanna Patent 1,828,284; as an example of a .phonograph pickup I refer to patent application lSerial No. 47,658, Sinnett et al., filed October 31, 1935, and issued as No. 2,105,167, same assignee, showing a hum neutralizing coil for neutralizing hum pickup by the pickup structure from'an external source such as a motor. While I have shown a particular type of neutralization means in a loudspeaker, I desire it to be understood that the invention may readily be applied to other arrangements, e.) g., wherein a neutralization coil is mounted externally of the speaker and coupled to a coil in series with the field coil, as shown in Fig. 3 of Pridham, -et al., 1,105,924. This method may be simplified by providing an extra winding on the usual output transformer located between s'aid field coil and said neutralizing coil, said ring being at least coextensive with said neutralizing coil and positioned closely adjacent thereto and said ring covering the entire area of the neutralizing coil facing said iield winding, whereby the opposing action of the neutralizing coil is obtained without the induction of disturbing voltages therein by the second part of said ilux.

2. In a sound transducer, a magnetic structure including a central pole piece, an apertured pole piece forming with one end of said central pole piece an annular air gap, a yoke member forming a magnetic path from the other end of said central pole piece to said apertured pole piece, an armature coil in said air gap, an annular neutralizing ,coil connected with the armature coiland having an inner diameter greater than the outer diameter of the armature coil, ysaid neutralcoil on said central pole piece adapted to be to the voice coil, and connecting this winding with the eld coil to thereby introduce an opposing E. M. F. into the voice' coil circuit.v

Although I have shown and described certain specific constructions, I want it understood that my invention is not limited'to the details set forth, It is expected that changes and modificationswill occur to those skilled in the art, without departing from the scope o1' the invention as dey lined in the appended claims,

piece an annular air gap, a yoke member form-l ing a magnetic path from the other end of said central pole piece to said apertured pole piece,

an 'armature coil in said air gap, an annular energized by fluctuating direct current which sets up uctuating ilux, comprising two parts, one part linking both the armature coil and the neutralizing coil and the other part not linking the armature coil but tending to link the neutralizing coil, the relative strengths of said parts varying with the frequency of the fluctuations of said flux, said armature and neutralizing coils being so poled that the voltages induced therein by the rst part of said ilux arein opposition, and a low resistance metal ring for shielding said neutralizing coil from said other part of the flux located between said i'leld coil and said neutralv izing coil, said ring being at least coextensive with said neutralizing coil and positioned closely adjacent thereto, and said ring covering the entire area of the neutralizing coil facing said field winding, whereby the opposing action of the neutralizing coil is obtained without induction of disturbing voltages therein by the second part of said iiux.

3. In a sound transducer, a magnetic structure l including a yoke, an annular pole piece and a neutralizing coil connected in series, with the armature coil and disposed around-the central pole piece adjacent the apertured pole piece, eld coil-on"said central pole piece adapted to be energized by iluctuating direct current which sets up iluctuating flux, comprising two parts, one part linking both the armature coil and the neutralizing coil and the other part not linking the armature coil but tending to link the neutralizing coil, the relative strength of said parts varying with the 'frequency of the fluctuations of said ux, said armature and neutralizing coils central pole piece forming an airgap therewith,

a source of iiuctuating magnetic ilux for said structure, a movable armature coil operatively disposed in said gap, a spacing ring of non-magnetic material secured to said annular pole piece and engaging said central pole piece for 'determining and maintaining said airgap, a neutralizingcoil connected in series opposing relation with said armature coil and disposed around said central pole piece, and means of non-magnetic low resistance material comprising an eddy cur.- rent ring disposed between said neutralizing coil and said source of uctuating magnetic flux.

WILLIAM F. STEERS. l 

